Deep collimating slot



IN VEN TORS Stephen M MacNei/le & By Kenneth lZ/Vw/(enz/e S M M NEILLE ETAL DEEP COLLIMATING SLOT Flled May 5, 1947 Aug. 16, 1955 United States Patent Ofiice 2,715,693 Patented Aug. 16, 1955 DEEP COLLIMATIN G SLOT Stephen M. MacNeille, Oak Ridge, Tenn., and Kenneth R. MacKenzie, Vancouver, British Columbia, Canada, assignors, by direct and mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application May 5, 1947, Serial No. 746,032

7 Claims. (Cl. 313-63) Our invention relates to ion sources, and more particularly to ion producing mechanisms for electromagnetically operated equipment for the separation of isotopes of elements.

It is generally the practice in the separation of isotopes of elements with electromagnetically operated equipment to first vaporize a charge material through heating, then lead the vapors to an ionizing chamber where they are subjected to electron bombardment to produce ions, eject the ions from the chamber with accelerating potentials, and cause them to travel through a magnetic field in arcuate paths whose radii correspond to their respective masses, and then collect the ions at or near the focal points of the ion beams.

The ionizing source usually takes the form of an electric are parallel to the magnetic field of the equipment between a hot filamentary cathode and the inside of an arc chamber. This are is comprised of ions and electrons in equilibrium forming a plasma in vapor containing the desired type of atom. The defining slot is formed in a separate plate next adjacent to the cathode, separating the cathode from the main part of the arc. Since the electrons in such a system can move essentially only parallel to the magnetic field, if the slot in the defining slot plate is smaller than the cathode, and opposite the cathode with respect to the magnetic field, the shape of the defining slot efiectively defines the cross section of the are beyond it. The defining slot and plate also perform the additional function of reducing the vapor in the region of the cathode and reducing the ion bombardment.

As originally conceived, the conventional ion source employed a rectangular defining slot opposite a straight section of cathode. The electrons passing through the defining slot then formed an arc of rectangular cross section of the same shape as the slot itself. It was from the plasma surrounding this arc column that the ions forming the beam were accelerated by the accelerating voltages of the equipment. In this arrangement the defining slot was usually formed by milling a slot in a plate just thick enough to provide a sufiiciently rigid member for the are system.

In these systems one of the major sources of stoppages or breakdowns of the equipment was the failure of the cathode element. This necessitated the closing down of the equipment and the replacement of the cathode. To bring the unit back into full operation again, hours of preliminary conditioning operations were required. The prime cause of the failure of cathodes is from ion bombardment. Flow of electrons from the cathode through the plate and defining slot across the ionizing chamber results in a flow of ions from the ionizing chamber through the defining slot to the cathode, which has the effect of striking an are from the cathode across the ionizing chamber. The ions flowing to the cathode to be neutralized, strike it at high velocities and accelerate its failure.

Applicants with a knowledge of these problems in the prior art have for an object of their invention the provision of an ion source having an improved ion barrier for reducing the bombardment of the cathode, and for increasing its useful life.

Applicants have as another object of their invention the provision of an ion source whose cathode is subjected to reduced vapor pressure to increase its useful life and reduce stoppages resulting from cathode failure.

Applicants have as a further object of their invention the provision of an ion source with an electron emitting cathode protected from excessive ion bombardment by a thickened cathode barrier, while preserving the desired cross section of the are set up in the ionizing chamber.

Applicants have as a still further object of their invention the provision of an ion source having a thickened plate with a deep defining slot therein to reduce the flow of ions and the bombardment of the electron emitting cathode by ions from the ionizing chamber.

Other objects and advantages of our invention will appear from the following specification and accompanying drawings, and the novel features thereof will be particularly pointed out in the annexed claims.

In the drawings, Fig. 1 is a perspective of a box construction defining an ionizing chamber with our improved cathode barrier incorporated therein. Fig. 2 is a cross section of the same construction taken along line 2--2 of Fig. l.

Failure of the cathode results from ion bombardment. Ions travelling at high speeds strike the cathode, knocking off pieces thereof. This action is What is commonly known as sputtering, and causes cathode failure. Additionally Where such elements as uranium are included in the vapors, the uranium reaching the filament tends to alloy with the tantalum or the tungsten thereof, and this alloy has a much lower melting point than the tantalum or tungsten of the filament, forming a liquid which tends to fiow at cathode operating temperatures. This results in early failure or breakdown of the filament. However, both of these actions are taking place simultaneously. At the higher voltages the former action predominates, while at the lower voltages, the latter action, which is much more rapid than the former, predominates. It was discovered that the use of a deep slotted barrier tended to lower the critical voltage at which the former action predominates as this former action is less destructive or acts more slowly than the latter action, the lowering of this critical voltage of predominance is helpful in extending the life of the cathode.

Referring to the drawings in detail, 1 designates a box of substantially parallelepiped configuration whose walls define an ionizing chamber 2. Charge in an adjacent or an adjoining chamber (not shown) is heated and the vapors are allowed to flow from the charge chamber to the ionizing chamber. The size and shape of the charge chamber may be conventional or of characteristic type, and vapors may be fed to the ionizing chamber 2 through a controlled port or orifice in the rear wall 3 of the box 1 in conformity with usual practice. The forward end of the box 1 may have electrodes 4, 5 with tapered edges 6, 7 mounted thereon in spaced relation to define a slit 8. The lower end 9 of box 1 may be in the form of a plate and serve as the anode. The upper end 11 of box 1 is open at 1.0 and a plate 12 with a defining slot 13 therein may be mounted over the opening. Carried by the upper end 11 are insulators 14, 14 which serve to mount the filament 15 over plate 12 so that the intermediate portion 16 thereof is in alignment with the defining slot 13 in the plate 12. Leads 17, 17 supply the filament with heater current.

It will be noted that the plate 12 has a substantially rectangular-shaped defining slot 13 therein, and the thickness of the plate has been increased far beyond that required for structural purposes. Preferably, the plate thickness is such that the depth of the slot 13 is at least five times its width. v

In its operation, cathode 15 is at a negative potential .with respect to plates 9 and 12, and electrons are emitted ftherefrom. i They are accelerated throughdefining slot 13 @and travel across chamber 2 to anode 9. 7 Neutral gases are fed into ionizing chamber 2 from the charge chamber 1(n'ot'shown). The bori 1 is disposed within the usual from cathode 15 'to anode 9 is substantially parallel thereto. Electronsin this path bombard the atoms of the neutral gas in chamber 2 and ionize it." As ions are formed,'they rush back toward the cathode, neutralizing the space charge about the cathode and permitting a rush {of electrons from the cathodeinto the arc-chamber. jUnder' these conditions the How of charged particles js trikes an arc and this are forms a'plasma where the particles are in dynamic equilibrium.

' Ions of substances which combine with the filament material at its'operating temperature are likely to be more harmful than other types of ions, and hence when several types of ions are present in thearc it would be The deep slot apparently has a tendency to discriminate .i between heavy and light ions flowing to the cathode in favor of the latter. In cases where these are less harmful to the cathode material,- -improved life is to be expected. 'In order, therefore, to reduce the probability of suchions reaching the cathode, a very thick barrier I or plate 12, having a deep slot 13 formed therein, is eml ployed to protect the cathode by positioning it betwen the cathode 15 and the ionizing chamber of box 1. The ions travelling in helical or spiral paths through the slot 13 toward the cathode 15 engage the walls of the slot and are either stopped, deflected, or their speed is so reduced'that they no longer strike the cathode with such velocity as to cause rapid breakdown or disintegration thereof. The thickened barrier serves the purpose of placing the cathode in a region of relatively low vapor pressure while the ionizing chamber is maintained at high pressure for efiective operation. This reduced vapor pressure at the cathode tends-t reduce disintegration and p, lengthen the life of the cathode; It may be further j pointed out that the plate or block 12 acts as a grid element, shaping the arc andinfluencing and regulating the flow of electrons to the anode 9. Use of the deep slotted block barrier to replace the thin plate previously used has resulted in a very marked reduction in cathode failures in production units. 7

Having thus described our invention, we claim: 7 l. An ion producing mechanism of the character described comprising an ionizing chamber adapted to receive gaseous vapors, a cathode positioned outside of said chamber for striking an arc across it to ionize said vapors, and means providing a thickened barrier for interposition between said cathode and said chamber to reduce cathode failurefrom bombardment, said barrier having an opening therein of less size than the thickness of said barrier.

scribed comprising an ionizing chamber adapted to receive gaseous vapors, a cathode for supplying a stream of electrons for ionizing said vapors, means providing a thickened barrier between said cathode and said ioninzing chamber to reduce cathode failure from ion bombard- ,magnetic field of the instrument so that the electron path well to choose, if possible, which type goes'to the cathode.

2. An ion producing mechanism of the character desaid barrier having a slot therein of such proportion that a the thickness of said barrier is a plurality of timesgreater than the width of said slot to reduce cathode failure from ion bombardment, an opening in said ionizing chamber, and electrodes positioned on either side of said opening to provide an accelerating potential for the ejection of ions from said ionizing chamber.

4. An ion producing mechanism of the character described comprising an ionizing chamber for the reception of gaseous vapors, a cathode for supplying a stream of electrons for ionizing said vapors, and athickened bari rierhaving a deep slot therein for passage of said electron stream while reducing cathode failure from ion bombardment, said barrier having a thickness at least five times the width of said slot.

5. An ion producing mechanism of the character. described comprising an ionizingchamber adapted to receive gaseous vapors, a cathode positioned outside of said chamber for striking an arc across it to ionize said vapors, and a thickened barrier having a deep slot therein interposed between said cathode and said ionizing chamber to permit the flow of electrons while lowering the critical alloying voltage of the cathode and'reducing ion bombardment, said slot having a width which is small in relation to the thickness of the barrier. V

6. An ion producing mechanism of the character described comprising an ionizing chamber adapted to receive gaseouszvapors, a cathode for feeding electrons into said chamberto ionize the vapors, a deep' slotted thick.-

ened barrier interposed between said cathode and said ionizing chamber to permit the flow of electrons while lowering the critical alloying voltage of the cathode and reducing ion bombardment, said'barrier having a thickness at least five times the width of said slot, an opening for said ionizing chamber, and electrodes positioned on either side of said opening to providev an accelerating potential for the ejection of ions from said ionizing chamber.

7. An ion producing mechanism of the character described comprising walls defining a box-like construction with an ionizing chamber adapted to receive gaseous vapors, acathode positioned outside of said chamber for feeding electrons thereto, a thickened barrier defining one of the walls of the chamber, a slot deepin relation to its width formed in said barrier, said barrier'permitting the flow of electrons through said slot while lowering the critical alloying voltage of the cathode and reducing ion bombardment, an opening in'said Walls communicating with said chamber, and electrodes positioned .-on either side of said opening to provide an accelerating potential for the ejection of ions from said ionizing chamber.

2,221,467 Bleakney Nov. 12, 19 40 Langmuir Mar. 6, 1945 i 

6. AN ION PRODUCING MECHANISM OF THE CHARACTER DESCRIBED COMPRISING AN IONIZING CHAMBER ADAPTED TO RECEIVE GASEOUS VAPORS, A CATHODE FOR FEEDING ELECTRONS INTO SAID CHAMBER TO IONIZE THE VAPORS, A DEEP SLOTTED THICKENED BARRIER INTERPOSED BETWEEN SAID CATHODE AND SAID IONIZING CHAMBER TO PERMIT THE FLOW OF ELECTRONS WHILE LOWERING THE CRITICAL ALLOYING VOLTAGE OF THE CATHODE AND REDUCING ION BOMBARDMENT, SAID BARRIER HAVING A THICKNESS AT LEAST FIVE TIMES THE WIDTH OF SAID SLOT, AND OPENING FOR SAID IONIZING CHAMBER, AND ELECTRODES POSITIONED ON POTENTIAL FOR THE EJECTION OF IONS FROM SAID IONIZING CHAMBER. 